Building wire is widely used to carry electrical current to all external uses of power in industrial, residential and commercial buildings. THHN (Thermoplastic High Heat-resistant Nylon coated) building wire is one type of wire or cable commonly used for this purpose. Typically, THHN building wire comes in stranded or solid conductors depending on the size. For example, solid conductors may include copper or aluminum, which are then covered in PVC (polyvinyl chloride) insulation with a nylon jacket or sheath. THHN building wire may also be used for wiring of machine tools, control circuits or on certain appliances. Commonly known cables used in building wire include, but not limited to Types THHN, THWN, THWN-2 type wires, 14-4/0 AWG, and 250-2000 kcmil sizes.
The prior art discloses that electrical cables such as THHN building wire generally include a conductor core and also an outer jacket or sheath. The term “sheath” is defined to mean the outermost protective jacket or covering surrounding a conductor core, whether of a single type material or multiple layers of the same or different material. The conductor core may typically be, for example, a single metal wire, multiple small wires twisted together to make a “stranded” cable, or multiple insulated wires or other type electrical conductors acting together to serve a particular function (e.g., three-phase connection). The sheath may comprise one or more layers of polymeric or other material to provide physical, mechanical, electrical insulating and/or chemical protection for the underlying cable components. The exterior portion of the sheath may be made of a polyamide composition, also often referred to as nylon. For example, a THHN building wire may comprise a conductor core of a single solid or stranded conductor, surrounded by a layer of polyvinyl chloride (PVC) electrical insulation, covered by an outer layer of nylon.
Installation of electrical cable often requires that it be pulled through tight spaces or small openings in, and in engagement with, narrow conduits, raceways, cabletrays, or passageways in rafters or joists. This becomes problematic since the exterior surface of the cable sheath normally has a high coefficient of friction, therefore requiring a large pulling force. Moreover, installation parameters include maximum allowable cable pulling tension and/or sidewall pressure limits. Exceeding these limits can result in degradation of the cable, physical damage and inferior installation. Thus, the need exists for a cable or wire that has a relatively low surface coefficient of friction so as to allow the installer to more easily pull the wire through the spaces and openings without too large of a pulling force that would cause degradation or damage to the wire or cable.
To overcome this problem, the general industry practice has been to coat the exterior surface of the cable sheath or jacket with a pulling lubricant at the job site in order to reduce the coefficient of friction between this surface and the conduit walls or like surfaces, typically using vaselines or lubricants produced specifically, and well known in the industry, for such purpose, such as Yellow 77®. As used herein, the term “pulling lubricant” is defined to mean a lubricating material added after polymerization of the nylon polymer, and typically during extrusion or formation of the product, or coated onto the finished product, which reduces the coefficient of friction of the exterior surface of the sheath or jacket of the cable to facilitate the pulling of the cable.
The aforementioned industry practice of applying a pulling lubricant to the finished cable at the work site poses problems, principally due to the additional time, expense and manpower required to lubricate the finished cable surface at the job site as well as to clean up after the lubricating process is completed. Thus, the need exists to provide a cable or wire, or at least an exterior sheath or jacket for a cable or wire that does not require the application of pulling lubricants at the work site.
Alternative solutions have been tried but have been generally unsuccessful, including the extrusion of a lubricant layer over the extruded polymeric sheath during the manufacturing of the cable, or the application of granules of material to the still-hot sheath during the extrusion process, which granules are designed to become detached when the cable is pulled through the duct. These solutions not only require major alterations of the manufacturing line, but also result in a loss in manufacturing time, increased economic costs, and undesirable fluctuations in the geometrical dimensions of the cable sheaths or jackets.
At least one patent in the prior art has distinguished between what are referred to as “pulling lubricants” and what are “processing lubricants.” A pulling lubricant is a lubricant that is applied to the nylon polymer product and appears at the outside surface of the sheath or jacket of the cable and is effective in lowering the surface coefficient of friction so as to reduce the force necessary to pull the cable along or through building surfaces or enclosures. A processing lubricant is lubricating material is a lubricant that is applied to the nylon polymer composition to facilitate the cable or wire manufacturing process, such as the flow of polymer chains during any polymer compounding as well as during the extrusion processes while the polymer is in its molten or melt phase. Cable manufacturers have long used processing lubricants, such as stearic acid or ethylene bis-stearamide wax, as a minor component of the polymeric compound from which the cable sheath or jacket is formed. Because a processing lubricant is normally not effective except when the polymer is in this melt phase, the effect of a processing lubricant is essentially non-existent in the final hardened polymer sheath of the cable. Under current technology, even where there may be an excessive amount of the processing lubricant, a separate pulling lubricant would still be required to sufficiently reduce the cable sheath's or jacket's exterior surface coefficient of friction in order to minimize the pulling force necessary to install the cable. In both definitions, however, the lubricants are added to already polymerized nylon polymer compositions either during a post-polymerization compounding process, during production of the cables or wires, or after.
In light of the need for better lubricity of the cable or wire, attempts have been made to provide building wire cables, and particularly the nylon cable outer sheaths and jackets used in building wire, with more slickness, i.e., a lower coefficient of friction, such that less externally applied pulling lubricants are required in order to minimize the pulling force necessary to install the building wire. In fact, it would be highly desirable if no externally applied pulling lubricants were necessary to pull the building wire, thereby saving costs, the mess created by using externally applied pulling lubricants, and operator strength for pulling the cable or wire.
Accordingly, attempts have been made to produce a cable or building wire with a sheath or jacket made of nylon material into which a lubricating agent, such as a silicone oil-based or an erucamide-based lubricant, is introduced. By doing so, the lubricity of the polymerized nylon cable, or stated another way, the coefficient of friction of the nylon cable or wire is lowered. That is, the lower the coefficient of friction, the more lubricity and more slickness is provided to the cable or wire. As used herein, a “lubricating agent” is a lubricant that is added to a nylon polymer composition after polymerization of the polyamide-producing monomers into nylon or other polyamides, whether added before, during or after the formation of product. Thus, by this definition, both pulling lubricants and processing lubricants, as well as any other lubricants that are added to the nylon polymer composition in any post-polymerization process, are considered lubricating agents.
One well known method for adding lubricating agents, e.g., silicone oil-based lubricants or erucamide-based lubricants, to the nylon material is accomplished by compounding the lubricant with remelted nylon polymer pellets in a post-polymerization extrusion process. Cables having outer sheaths or jackets produced via this process have been found to have a reduced surface coefficient of friction sufficient to lower the required installation pulling force of the cable, such that the need for externally applied pulling lubricants can be greatly reduced or even eliminated. Unfortunately, it has been found that, in order to be effective in lowering the coefficient of friction and to sufficiently increase the slickness of the cable (and more particularly, its sheaths) via this method, high loadings of at least 9% by weight of lubricating agents are required. While improving surface lubricity, such higher loadings of lubricating agents can compromise cable processing efficiency, can reduce protective sheath mechanical properties and can reduce flame retardancy, all of which compromise the desired characteristics of the cable sheath.
Thus, the need exists for improved nylon compositions having a low surface coefficient of friction, and particularly, compositions having a polyamide matrix suitable for use in building wire as slick cables or cable sheaths that do not require high levels of lubricating agents to be added during any post-polymerization process. Heretofore, no one has successfully produced a nylon polymer composition that has achieved this need for the slick cable and building wire industry.
Silicon dioxide, commonly known as silica, is an inorganic material derived from quartz and other rock like minerals. In one instance, namely, in the production of bi-axial oriented polyamide film compositions for the film packaging industry, it has been found that the intimate dispersion of very low amounts (i.e., 2.0 wt. % or less) of silica having a particular particle diameter (of 0.5 to 5 μm) during the preparation of a polyamide film can provide a nylon polymer composition having excellent slipperiness. For film producers, the advantage of polymer film containing particulate silica is to avoid film blocking, where adjacent (on top of each other) layers often stick to each other. Having a nylon composition with increased slipperiness helps to avoid this problem.
However, the use of greater amounts of silica or larger particle sizes of silica was found to be problematic in that the greater amounts of silica or larger particles was shown to create aggregation (i.e., agglomeration of particles) and migration of the particles to the surface of the films. This was noted to cause film breakage during processing and film aesthetic defects, inasmuch as the films were required to be clear, but aggregation and migration of silica is well known to cause clarity issues in films. Further, it was found that larger particles of silica and/or high loadings of silica have a tendency to clog filters during processing, to increase the likelihood of caking during processing, and to provide overall bad workability for the nylon polymer composition, thus making the addition of silica not attractive for other commercial purposes. Thus, only very small amounts and very small particles of silica were shown to be suitable for use in the limited field of bi-axial oriented polyamide film.
Nevertheless, if a nylon polymer composition could be made with particulate silica having much larger particle sizes (significantly larger than those used in the nylon compositions for anti-blocking films), or alternatively, in sufficient loadings (significantly higher than those used in nylon compositions for anti-blocking films) added in before the polymerization process to provide inherent lubricity to the nylon polymer composition itself, such nylon polymer compositions would be highly desirable in any of a number of industries, including the slick cable and building wire industry. If the surface coefficient of friction of the nylon polymer composition after polymerization was lowered sufficiently enough, no additional lubricating agents would be needed, and even reducing the amount of lubricating agents needed would be beneficial to many industries.
Such desirable nylon polymer compositions could use silica or other lubricant additives to obtain lubricity and not require lubricating agents, such as silicone oil, in them. As the terms are used herein, there is a distinction to be made between “lubricating agents” and “lubricant additives.” As used herein and as defined above, lubricating agents include both pulling lubricants and processing lubricants, as well as any other lubricants that are added to the nylon polymer composition whether added before or after the formation of the product, but after the polymerization process for producing the nylon polymer compositions have occurred. In contrast, lubricant additives are defined as lubricants that are specifically mixed with at least one polymer-producing monomer and water prior to polymerization, such that the resultant polymer, here, polyamide (nylon) polymer, contains the lubricant additive in situ, within the nylon polymer composition upon polymerization. Such polymers are made preferably with particulate silica well-dispersed (i.e., homogeneous) within the polymer backbone. Such polymers provide the advantage of being suitable for the production and manufacture of slick cables or cable sheaths or jackets for building wire. The silica may be either chemically or mechanically bound within the polyamide matrix.
In addition to the incorporation of silica, it is believed that the incorporation of lubricant additives such as mineral oil would also be desirable. However, heretofore, the introduction, let alone the substantially uniform dispersion, of lubricant additives into a polyamide matrix prior to polymerization has never been successful due to the “oil like” nature of the lubricant additives. Introduction of lubricating agents in a subsequent, post-polymerization extrusion/compounding process has been well documented, but high levels of lubricating agents are required due to the lubrication method and the lack of synergy achieved between the lubricating agents and the polyamide matrix. Moreover, the introduction of silica in the post-polymerization process does not create sufficient synergy or lower the surface coefficient of friction sufficiently enough to allow even a significant reduction in the amounts of the lubricating agents used.
Thus, the need continues to exist for a process by which a lubricant additive, such as mineral oil, can be added to a nylon polymer composition. Such a process will necessarily need to be able to substantially uniformly disperse the lubricant additive, as well as any other component, e.g., silica, throughout the polymer matrix in order to achieve the desired surface lubricity necessary for satisfactory use as the sheath or jacket of a wire or cable. Such a process would satisfy a long felt need for cost effective manufacturing of polyamides suitable, for example, in the forming of slick cables and other applications. However, currently, the best alternative is to add high loadings of lubricating agents during the production of the cable or wire sheaths or jackets, a well known post-polymerization process.